RF receivers incorporating analog or digital automatic gain controllers (AGC) are well-known and widely used. Although the applications in which RF receivers with AGC are used are too numerous to describe in detail, one increasingly popular application is in the field of wireless computer systems, such as wireless LANs.
In a typical wireless computer network environment, the "backbone" of the LAN takes the form of one or more central servers that communicate with a number of network base stations, or access points ("APs"), through a hard-wired connection. Each access point (AP) includes an RF receiver/transmitter for communicating with at least one roaming mobile station ("MS"), which also incorporates an RF receiver/transmitter. These RF receivers often employ either direct sequence spread spectrum ("DSSS") or frequency-hopping spread spectrum ("FHSS") techniques to communicate between roaming mobile stations and network access points. The mobile station may be a point-of-sale terminal (e.g., an electronic cash register), a bar code reader or other scanning device, or a notepad, desktop or laptop computer.
Each MS establishes a communication link with an AP by scanning the ISM (industrial, scientific, medical) band to find an available AP. Once a reliable link is established, the MS interacts with other mobile stations, network servers, or a combination thereof. This allows the user of the MS to move freely in the office, factory, hospital or other facility where the wireless LAN is based, without the length of a hard-wired connection to the LAN limiting the MS. This communication link is maintained even if the MS moves out of range of its current AP. When the MS senses that the communication link with the current AP is unacceptably weak, the MS initiates a scanning process that ultimately results in a "handover" that establishes a new communication link with a second AP. Ideally, this handover is "seamless" to the user. That is, the handover is performed from one AP to the second AP without requiring user input and without interrupting an on-going task in the MS.
The RF receiver in the mobile station typically incorporates an automatic gain control (AGC) circuit that adjusts the strength of the received signal to an optimum level within the dynamic range of various signal processing components in the received signal path. The AGC circuit initially helps to bring a newly received signal into the dynamic range of the receiver when the MS is first establishing a communication link with an AP. Thereafter, the AGC circuit continually adjusts the receiver gain as data packets are received from the AP in order to compensate for fluctuations in received signal strength associated with fading, interference, periods of "silence" between data frames, etc.
The "settling" time of an AGC is the time required by the AGC to bring the received signal to an optimum level within the dynamic range of the RF receiver. During the settling time, the RF receiver is generally unable to perform other functions, such as training, symbol detection, etc. This means that the settling time tends to increase the "overhead" of the RF receiver. Since the RF receiver is part of a high-speed communication link, it is extremely important to minimize the settling time of the AGC.
A wide variety of AGC systems are known in the prior art. These AGC systems include both analog and digital designs, and vary considerably according to speed, accuracy, cost and complexity. Analog systems frequently operate at higher speeds (i.e., very short settling times) than corresponding digital systems. However, analog systems are generally more complex, and therefore more costly to implement, than equivalent digital designs.
Therefore, there is a need in RF receiver technology for improved systems and methods of automatic gain control that provide very high-speed adjustment of RF receiver gain. In particular, there is a need for improved digital AGCs for use in digital RF receivers that rapidly bring the strength of a received input signal to a desired target level in the dynamic range of the RF receiver.